Radio frequency (RF) modules that are used for mobile radio applications are constructed on multilayer substrates with interconnections integrated therein. A conventional transmission module may include a transmission amplifier (power amplifier), which is disposed at the top of the substrate module as a naked chip (bare die) or as a housed element. The module can include a matching network for transforming the output impedance of the power amplifier to the input impedance (usually 50 ohms) of the next element in the vicinity of the circuit.
A further level of integration includes so-called power switch modules PSM (also called TX modules), in which, in addition to the output matching network, TX filters and transmission/reception switches are provided. These are located as discrete elements on a top side of the module substrate.
The highest level of integration is represented by so-called TX modules with integrated reception filtering (also called power switch modules with integrated filtering, PSMiF). In such modules, reception filters are integrated as discrete elements on the top side of the module substrate and are connected with, or integrated in, the module.
An Rx signal path and an Rx output, at least one Tx signal path, at least one Tx input, and an antenna connector, along with to any number of other RF components and associated ICs, can be located on, or integrated in, a dielectric substrate of the RF module.
There are, however, requirements on the part of chip and telephone manufacturers to implement an RX path for future Universal Mobile Telecommunications System (UMTS) systems in a balanced manner starting with the duplexer. Such a symmetrical output requires two connectors, at which two signals of same magnitude but of opposite polarity, i.e., ideally with a 180° phases shift, can be obtained. Such balanced outputs are particularly simple to be supplied to modern RX signal amplifiers. The terminals used for Global System for Mobile Communications (GSM) also require a symmetrical Rx signal.
A balanced output signal cannot be provided directly by a microwave ceramic filter or duplexer and therefore requires an external balun. Other filter techniques, such as FBAR filters and duplexers or SAW filters and duplexers can maintain the balun functionality in the integrated filter structure. External baluns are configured as discrete elements and are, for example, located on the RF module or the front-end module.